Do you have to worry about using long cable runs with transformerless mics?

What are some of your favorite tranny-less mics??

Why did Bambi's mom have to die?

Thanks, Bubba

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"When we make music we don't do it in order to reach a certain point, such as the end of the composition. If that were the purpose of music then obviously the fastest players would be the best. Also, when we are dancing we are not aiming to arrive at a particular place on the floor as in a journey. When we dance, the journey itself is the point, as when we play music the playing itself is the point." -Alan Watts

Well, there's a lot of different mic types, but here's a few explanations:

First, many dynamic mics produce so little voltage (Shure SM57/58, Senheiser 412, AKG D12, etc) that any induced noise is significant compared to the signal. A step-up transformer makes the voltage HIGHER in these cases, while not making the impedance too ridiculously high.

Tube condenser microphones on the other hand frequently produce larger signals at the anode or cathode of the tube, so the signal-to-noise voltage ratio (in terms of surviving a long trip down a mic line) is pretty good... however, the impedance might be too high for even moderate length cables (the higher the source impedance, the more high end is lost to the inevitable cable capacitance, which increases proportional to cable length). -In addition, the tube produces a single, unbalanced signal. A transformer also has the wonderful benefits of galvanic isolation AND balancing.

Now, for phantom powered microphones, the simplest circuits are something like the KM84, where a single FET is used in a comparable way to a tube in a simple tube mic. It produces a single unbalanced output signal, and the transformer effectively balances the signal, as well as tweaking the impedance to suit the typical range of mic preamp loads.

Now... on to transformerless mics:

There ARE transformerless tube designs. You don't encounter them much, but they do exist. -A friend of mine builds all of his this way. With these, you do have to be mindful of cable length.

Many designs however (like the much-imitated Schoeps), produce a differential output signal which is itself essentially balanced. Therefore part of the 'need' for an (expensive when done right) transformer is suddenly less pressing.

In addition, they use fairly low-impedance drive circuitry, which is capable of driving longer lines with little loss of high end (in the case of many transformerless Chinese mics, a little HF rolloff might actually be a NICE thing! -although that's a personal taste issue, of course).

Some transformerless mics produce an unbalanced signal, but use the 'silent' pin (from which they still draw half of their phantom power current) as a matched-impedance connection to ground, which allows most balanced-input mic preamps to still cancel noise fairly effectively.

-So there are a few designs out there, and they can work QUITE differently, but off the top of my head, I'd say that the only transformerless mic designs that spring to mind which WOULD be particularly sensitive to long lines are the transformerless cathode follower tube designs which I've seen... and they're not a common topology, in my experience.

It's a good thing to ask however. -While I'd say that most mics manufactured for general sale have been engineered to be capable of driving fairly long lines without great problems, there may well always be one which catches you out!

But as to whether being 'transformerless' has any automatic bearing on ability to drive long lines... I'd say basically that the answer is no.

Hope that helps more than it confuses the issue!

Keith

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MDM (maxdimario) wrote on Fri, 16 November 2007 21:36

I have the feeling that I have more experience in my little finger than you do in your whole body about audio electronics..

Oh, and another thing about transformers: -think of them like gearboxes.

When I was little I was fascinated by the fact that I could put a big cog on one axle, and a small cog on another, turn a crank and get the smaller one to turn MUCH faster than the crank which I turned. -It was like getting 'speed' for free.

Of course, (using 'Meccano') I used to then connect another large cog onto the second shaft, then turn a third shaft EVEN FASTER... then a fourth and so on.

One afternoon, I built a step-up gearbox with an input-ouptut ratio of about 100:1. -If I turned the input shaft at 1RPM, the last shaft turned at 100RPM. -But I realised how HARD it was to turn the input crank. -I noticed that I could make the crank IMPOSSIBLE to turn by just applying the SLIGHTEST braking force to the output. -My young brain was puzzled by why this 'free energy' (as I thought it was) was being overcome.

Of course, with any gearbox -like a car transmission, or a bicycle derailleur gear set- as you INCREASE the speed multiplication and increase the 'speed ratio' you DECREASE the 'torque ratio'. Conversely, as you DECREASE the speed ratio, you INCREASE the torque ratio.

This is because power equals speed multiplied by torque. Power cannot be 'created' out of nowhere (as I'd hoped as a 5-year old!) but CAN be transformed to a different ratio of speed versus torque.

In an AC transformer, Voltage is like 'speed' and current is like 'torque'. Double the transformation of one and you halve the availability of the other.

So, a transformer for a microphone might need to be a STEP UP (voltage multiplying) transformer, as in many dynamic mic designs -ribbon mics being an EXTREME example- or a STEP DOWN (voltage dividing)transformer, but it can ONLY produce enough voltage and current at a low enough impedance, if there is enough actual POWER present initially.

The IMPEDANCE is probably the most important factor which affects long line driveability. So long as this is nice and low, the mic should be able to push a decent length of copper. If it's balanced (either voltage balanced, transformer balanced or impedance balanced) then the receiving preamp should be able to reject most noise picked up along the way.

Keith

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MDM (maxdimario) wrote on Fri, 16 November 2007 21:36

I have the feeling that I have more experience in my little finger than you do in your whole body about audio electronics..

"When we make music we don't do it in order to reach a certain point, such as the end of the composition. If that were the purpose of music then obviously the fastest players would be the best. Also, when we are dancing we are not aiming to arrive at a particular place on the floor as in a journey. When we dance, the journey itself is the point, as when we play music the playing itself is the point." -Alan Watts

As to 'When did they start making them and why?' I'm not sure when the first ones would have been, but the 'why' was very likely a combination of cost (GOOD transformers aren't usually cheap, and CHEAP transformers aren't usually good!) and the fact that high ratio transformers sometimes can start to limit the extension of a flat frequency response. -It can be easier to build circuits with ruler-flat response graphs without high-ratio iron in the circuit.

For a while there, the dual siren temptations of better paper-spec graphs (which the sales/marketing guys like) and lower manufacturing costs (which the bean-counter/production costing guys like) made a number of designs centered around transformerless topology highly interesting to manufacturers.

Unfortunately one man's "extended HF response" is another man's "harsh".

-And as for Bambi's mother, that ho had it comin'.-You jus' can't let it slide when a bitch starts holdin' out on y'all... -elsewise they ALL gonna start skimmin' yo Benjamins. -Hafta make an example o' tha first bitch dat step outta line, -dig?

Keef

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MDM (maxdimario) wrote on Fri, 16 November 2007 21:36

I have the feeling that I have more experience in my little finger than you do in your whole body about audio electronics..

While Keith's abundant explanation is both generous and impressive, if I may, I'd like to give my side now:

Many microphones {tranny or not} are only good for what/who they are intended to be used for. This applies especially for singers.

IOW, a condenser mic can sound terrible on a singer while the dynamic mic may subjectively sound like a better choice. Naturally there are wide differences in condenser mics including those with tubes, but you get my point.

Knowing the right mic for the task at hand: priceless.

So, in selecting a microphone for recording or a live application, I believe their specs could mean very little.

You know, I really don't want to 'pile-on', but I fail to see where Ed's post addresses a single part of the original question, or any other question raised subsequently...

The point about many mics being 'only good for what/who they are intended to be used for' doesn't sit comfortably with me though... I know people who use a D12 round the back of an AC30. Many, many engineers LOVE to use a U47FET right in front of a kick drum... I really can't help thinking that this over-generalized point doesn't really help anyone much at all... unless anyone else can convince me otherwise.

I know this is Fletcher's forum, but... "Whatever works" really is an awfully good mantra. -I think that most people know this of course.

KNOWING 'what works' is the key... and FINDING 'what works' is of course made an awful lot easier with a combination of access to lots of options for experimentation, and "seat-time".

Now I appreciate that I did introduce various TYPES of microphones into my initial reply, but specifically to show how some designs (many moving coil, and ALL ribbon for example) need step-up trannys, while others (many tube microphone designs) use step DOWN trannys, because the balance of the 'problems' in each case varies significantly, and this affects impedance transformation (impedance ratio varies as the SQUARE of the voltage transformation ratio), and impedance is invariably the governing factor where people become troubled by the effect of long lines on microphone signals.

So while I felt obligated to consider various fundamental topologies, it was never to suggest any limitation of what can be used where; only to try to give a more comprehensive appreciation of the 'long-line-detriment' mechanism.

Oh, and thank you, Peter... I've enjoyed reading many of your posts on microphones over the years, and have a great appreciation for your own posts in the past.

Keith

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MDM (maxdimario) wrote on Fri, 16 November 2007 21:36

I have the feeling that I have more experience in my little finger than you do in your whole body about audio electronics..

Already asked by the OP, and none of your posts addressed any of his questions.

However, please enlighten us about a few points raised recently by you in this and another thread:

- do specs matter when choosing a piece of equipment during a recording session, or should the engineer rely on his knowledge and experimentation to decide which one to use in order to achieve the sound that the client is after and/or that will better serve the sonic goals at hand?

- your response to the above applies only to microphones or does it apply to signal processing gear too?